Single crystal seed

ABSTRACT

A single crystal seed for use in casting a single crystal article, consisting essentially of, in weight %, about 5.0% to about 40.0% Mo, up to 0.1% C and balance essentially Ni.

FIELD OF THE INVENTION

The present invention relates to single crystal casting using a seedcrystal in a ceramic shell mold.

BACKGROUND OF THE INVENTION

Single crystal casting and solidification of molten metal using aceramic investment shell mold positioned on a chill plate is well knownin the art of investment casting to produce such components as singlecrystal gas turbine engine blades and vanes. In one mode of singlecrystal casting, the ceramic shell mold includes a lower grain startercavity having a single crystal seed positioned therein. The seed iscommunicated to an upper mold cavity. The seed is placed in or otherwisecommunicated to the lower starter cavity of a ceramic shell mold that ispreformed using the well known lost wax process. When molten metal isintroduced into the shell mold, a single crystal is expected to growepitaxially upwardly from the seed with a preselected crystallographicorientation determined by the seed for propagation through the moltenmetal in the mold cavity. During single crystal solidification, themelt-filled shell mold is withdrawn from a casting furnace to establishunidirectional heat removal from the molten metal in the mold to producea solidification front of the single crystal through the molten metal inthe mold cavity to form the single crystal casting.

Single crystal seeds used heretofore in casting nickel base superalloyshave employed seed alloy compositions similar to that of the nickel basesuperalloy to be cast. Such seeds can experience oxide growth on theseed surfaces at the elevated mold preheat temperatures employed insingle crystal casting. This oxide growth occurs while preheating themold in a vacuum casting furnace and can result in nucleation ofspurious grains that grow in the starter cavity instead of the expectedsingle crystal. Such spurious grain nucleation and growth in the startercavity can result in an increase in scrap castings by virtue of theirhaving a grain orientation outside a preselected specification (e.g.where the [001] crystal axis must be within a certain range of degreesrelative to the z axis of the turbine airfoil).

An object of the present invention is to provide a single crystal seedas well as casting mold and method for making a single crystal castingthat overcome the above problem of oxide growth and spurious grainnucleation and the need for special gating schemes.

SUMMARY OF THE INVENTION

In an embodiment of the present invention, a single crystal seed isprovided for casting nickel alloys and cobalt alloys as single crystalswherein the seed comprises one or more alloying elements selected fromthe group consisting of Mo, Cr, W, Ta, Re, Nb, and V in certain amountsand the balance Ni and/or Co. The seed can be used to cast singlecrystal nickel base superalloy articles such as turbine blades and vanesas well as nickel alloy and cobalt alloy seeds for use in single crystalcasting of nickel base superalloys.

For example, in an illustrative embodiment of the present invention, asingle crystal seed is provided consisting essentially of, in weight %,about 5.0% to about 40.0% Mo, up to 0.1% C and/or B, and balanceessentially Ni. The seed is resistant to oxide growth under singlecrystal mold preheating conditions to overcome the above problems ofoxide growth and spurious grain nucleation and the need for specialgating schemes.

The seed is positioned in a starter cavity of a single crystal castingmold to initiate epitaxial grain nucleation and growth in moltenmetallic material residing in an article mold cavity.

The above objects and advantages of the present invention will becomemore readily apparent from the following detailed description taken withthe following drawings.

DESCRIPTION OF THE DRAWINGS

The FIGURE is a sectional view of a ceramic investment shell mold havinga single crystal seed pursuant to an illustrative embodiment of theinvention residing in a grain starter cavity.

DESCRIPTION OF THE INVENTION

An illustrative embodiment of the present invention involvesoxide-resistant single crystal seeds for use in casting of singlecrystal nickel base superalloys as single crystal airfoils, such asturbine blades and vanes, of a gas turbine engine, and other articleshapes. Another illustrative embodiment of the present inventioninvolves oxide-resistant single crystal seeds for casting of nickelalloy seed bodies and cobalt alloy seed bodies that can be used insingle crystal casting processes to make other articles, such as theaforementioned single crystal nickel base superalloy airfoils.

Referring the FIGURE, a ceramic investment shell mold 10 includes asingle crystal seed 20 in the form a cast nickel alloy or cobalt alloybody 21 residing in a grain starter cavity 12 at the lower end of themold. The grain starter cavity 12 is communicated at its upper end by aconnector passage 14 to an article mold cavity 16 having the shape ofthe article to be cast. The article to be cast can be a gas turbineengine airfoil, such as a turbine blade or vane, or any other productshape. The article to be cast also can be a single crystal seed body 21itself to be used in single crystal casting of nickel base superalloyarticles. The pour cup 18 is provided above the mold cavity 16 to feedmolten metal or alloy to the article cavity and starter cavity. Thegrain starter cavity 12 is communicated at its lower end to aconventional chill plate 40 on a ram 42 of a conventional castingmachine that embodies the well known Bridgeman mold withdrawaltechnique. The seed 20 can sit directly on the chill plate 40 or bedisposed in the mold out of contact with the chill plate.

The single crystal seed 20 can be incorporated in the starter cavity 12using conventional shell mold fabrication techniques where a wax patternassembly comprising the starter cavity, connector passage, and articlecavity is formed and invested in ceramic shell mold material using thewell known lost wax process. After the pattern assembly is removed fromthe shell mold invested thereon, the shell mold is fired at an elevatedtemperature to develop mold strength for casting. The seed 20 then isinserted into the open end of the starter cavity 12. The seed typicallyis a cast cylindrical body 21 in shape, although other seed shapes orconfigurations can be used in practicing the invention. Alternatively,the single crystal seed 20 can be attached to the wax assembly and thenthe shell mold is built-up around the assembly and the seed 20.

The present invention envisions a seed 20 comprising one or morealloying elements selected from the group consisting of Mo, Cr, W, Ta,Re, Nb, and V in certain amounts and the balance Ni, Co or combinationsthereof. The listed alloying elements are precipitation hardeningalloyants that form precipitates with Ni or Co, or with one another orother alloyants, upon being subjected to a suitable precipitationhardening heat treatment. One or more of C and B may optionally bepresent in the seed 20.

Pursuant to an illustrative embodiment of the present invention, theseed 20 consists essentially of, in weight %, about 5.0% to about 40.0%Mo, up to 0.1% of an element selected from the group consisting of C andB and combinations thereof, and balance selected from the groupconsisting of Ni and Co and combinations thereof. A preferredcomposition of the seed 20 for casting nickel base superalloys andnickel alloy seed bodies 21 consists essentially of, in weight %, about15% to about 30% Mo, 0 to about 0.05% C and/or B, and balanceessentially Ni. A preferred composition of the seed 20 for castingcobalt alloy seed bodies 21 consists essentially of, in weight %, about8% to about 28% Mo, 0 to about 0.05% C and/or B, and balance essentiallyCo. The Mo content within the ranges set forth imparts oxidationresistance, solid solution strengthening, and precipitationhardenability to the seed 20. The C and/or B content within the rangesset forth imparts alloy cleanliness and/or liquidus/solidus temperaturecontrol to the seed 20. The seed 20 is resistant to oxidation and oxidegrowth at the high mold preheating temperature of typical single crystalcasting processing to overcome the above problems of oxide growth andspurious grain nucleation as well as the need for special gatingschemes. The mold 10 with the seed 20 of the invention can be filledwith molten superalloy through the upper pour cup 18 at its upper endwhile the mold resides in the vacuum casting furnace as is well known orby any conventional other mold filling technique.

In other illustrative embodiments of the present invention, the alloyingelement(s) can be selected from one or more of the group consisting ofMo, Cr, W, Ta, Re, Nb, V and combinations thereof in appropriateconcentration(s) in a nickel alloy or cobalt alloy seed 20 toprecipitation harden the seed upon suitable heat treatment.

For example, for purposes of illustration and not limitation, a nickelbased alloy seed 20 can comprise 1) about 15 weight % to about 47 weight% Cr and balance Ni and optionally up to 0.1 weight % C and/or B, 2)about 10 weight % to about 45 weight % W and balance Ni and optionallyup to 0.1 weight % C and/or B, 3) about 15 weight % to about 33 weight %Ta and balance Ni and optionally up to 0.1 weight % C and/or B, 4) about8 weight % to about 13 weight % Re and balance Ni and optionally up to0.1 weight % C and/or B, 5) about 5 weight % to about 18 weight % Nb andbalance Ni and optionally up to 0.1 weight % C and/or B, 6) about 5weight % to about 30 weight % V and balance Ni and optionally up to 0.1weight % C and/or B.

For purposes of further illustration and not limitation, a cobalt basedalloy seed 20 can comprise 1) about 2 weight % to about 37 weight % Crand balance Co and optionally up to 0.1 weight % C and/or B, 2) about 10weight % to about 40 weight % W and balance Co and optionally up to 0.1weight % C and/or B, 3) about 5 weight % to about 44 weight % Ta andbalance Co and optionally up to 0.1 weight % C and/or B, 4) about 5weight % to about 36 weight % Re and balance Co and optionally up to 0.1weight % C and/or B, 5) about 1 weight % to about 12 weight % Nb andbalance Co and optionally up to 0.1 weight % C and/or B, 6) about 2weight % to about 28 weight % V and balance Co and optionally up to 0.1weight C and/or B.

The seed 20 is provided with a crystallographic orientation to beimparted to the single crystal casting in the article cavity 16 byepitaxial solidification therefrom as is well known. For example only,for most face centered cubic nickel base superalloys, the seed crystalcan have a crystal axis oriented parallel to the z axis of a turbineairfoil to be cast in the mold cavity 16. The seed 20 can be formed withthe desired crystallographic orientation by casting the seed 20 with thedesired orientation, or the seed can be physically oriented to provide acasting with a desired orientation relative to the mold cavity 16.

The seed 20 can be used in the as-cast seed condition or optionally canbe precipitation hardened after casting by heat treatment prior toplacing the seed 20 in the shell mold. The heat treatment is conductedat temperatures and times to form precipitates of one or more of theabove-listed precipitation hardening elements (e.g. Mo, Cr, W, Ta, Re,Nb, and/or V) with Ni or Co and/or with one another or other alloyantspresent. The temperatures and times employed for the precipitationhardening heat treatment will depend on the seed alloy composition used.The temperatures and times of the heat treatment can be determined fromphase diagrams of the particular seed alloy employed and/or empiricallyfrom heat treatment evaluation tests.

For casting single crystal nickel base superalloys and nickel and cobaltseed alloys, the shell mold 10 typically is heated in a vacuum castingfurnace 50 having a susceptor 52 inductively heated by induction coil 54to an elevated temperature in the range of 2700 to 2900 degrees F. priorto pouring of molten superheated alloy therein. The vacuum level(subambient pressure) in the casting furnace typically is less than 15microns. The temperature of the superheated molten alloy will depend onthe alloy being cast and generally is in the range of 200 to 500 degreesF. above the alloy melting point.

After the molten alloy is poured into pour cup 18, it flows downwardlyto fill the mold cavity 16 and the starter cavity 12, whereby the moltenalloy melts an upper region of the seed 20. The ram 42 is moved towithdraw the mold 10 from the casting furnace 50 to establish asolidification front in the molten alloy that progresses upwardly fromthe starter cavity 12 through the connector passage 14 and the articlecavity 16 in known manner to form a single crystal cast article thereinas described for example, in Ver Snyder U.S. Pat. No. 3,260,505 andPiearcey U.S. Pat. No. 3,494,709, which are incorporated by referenceherein.

Use of the oxidation-resistant seed 20 pursuant to the invention reducesor eliminates spurious grain nucleation in the starter cavity 12.Casting defects due to grain misorientation are significantly reduced bypractice of the invention.

The following Examples are offered to further illustrate, but not limit,the invention:

EXAMPLES

A nickel base superalloy known as CMSX-10 was superheated to 2750degrees F. and poured into mold 10 heated to 2825 degrees F. in a vacuumcasting furnace. The single crystal seed 20 comprised Ni-26.6 weight %Mo (i.e. 26.6 weight % Mo and balance Ni) and was precipitation hardenedprior to placement in the mold by heat treatment at 1400 degrees F. for6 hours and air cooled to ambient temperature. The seed did not exhibitsubstantial oxidation such that no spurious grain nucleation occurred inthe starter cavity 12.

CMSX-10 also was superheated to 3015 degrees F. and poured into a shellmold 10 heated to 2825 degrees F. in a vacuum casting furnace. The seed20 comprised Ni-27.52 weight % Mo and was precipitation hardened priorto placement in the mold by heat treatment at 1400 degrees F. for 6hours and air cooled to ambient temperature. The seed did not exhibitsubstantial oxidation such that no spurious grain nucleation occurred inthe starter cavity 12.

A nickel base superalloy known as Rene' N5 was superheated to 2700degrees F. and poured into mold 10 heated to 2825 degrees F. in a vacuumcasting furnace. The seed 20 comprised Ni-25 weight % Mo and wasprecipitation hardened prior to placement in the mold by heat treatmentat 1400 degrees F. for 6 hours and air cooled to ambient temperature.The seed did not exhibit substantial oxidation such that no spuriousgrain nucleation occurred in the starter cavity 12.

Rene' N5 also was superheated to 2700 degrees F. and poured into mold 10heated to 2825 degrees F. in a vacuum casting furnace. The seed 20comprised Ni-15 weight % Mo-0.05 weight % C and was precipitationhardened prior to placement in the mold by heat treatment at 910 degreesF. for 6 hours and air cooled to ambient temperature. The seed did notexhibit substantial oxidation such that no spurious grain nucleationoccurred in the starter cavity 12.

Rene' N5 also was superheated to 2700 degrees F. and poured into mold 10heated to 2825 degrees F. in a vacuum casting furnace. The seed 20comprised Ni-26.4 weight % Mo and was precipitation hardened prior toplacement in the mold by heat treatment at 910 degrees F. for 6 hoursand air cooled to ambient temperature. The seed did not exhibitsubstantial oxidation such that no spurious grain nucleation occurred inthe starter cavity 12.

A fourth generation single crystal nickel base superalloy wassuperheated to 2725 degrees F. and poured into mold 10 heated to 2825degrees F. in a vacuum casting furnace. The seed 20 comprised Ni-27weight % Mo and was precipitation hardened prior to placement in themold by heat treatment at 1400 degrees F. for 6 hours and air cooled toambient temperature. The seed did not exhibit substantial oxidation suchthat no spurious grain nucleation occurred in the starter cavity 12.

A nickel single crystal seed alloy comprising 20 weight % Mo and balanceNi was superheated to 2880 degrees F. and poured into mold 10 heated to2825 degrees F. in a vacuum casting furnace. The seed 20 comprised Ni-25weight % Mo and was precipitation hardened prior to placement in themold by heat treatment at 1400 degrees F. for 6 hours and air cooled toambient temperature. The seed did not exhibit substantial oxidation suchthat no spurious grain nucleation occurred in the starter cavity 12.

A nickel single crystal seed alloy comprising 20 weight % Re and balanceNi was superheated to 2900 degrees F. and poured into mold 10 heated to2825 degrees F. in a vacuum casting furnace. The seed 20 comprised Ni-25weight % Mo and was precipitation hardened prior to placement in themold by heat treatment at 1400 degrees F. for 6 hours and air cooled toambient temperature. The seed did not exhibit substantial oxidation suchthat no spurious grain nucleation occurred in the starter cavity 12

A nickel single crystal seed alloy comprising 25 weight % Mo and balanceNi was superheated to 2700 degrees F. and poured into mold 10 heated to2825 degrees F. in a vacuum casting furnace. The seed 20 comprised Ni-25weight % Mo and was precipitation hardened prior to placement in themold by heat treatment at 1400 degrees F. for 6 hours and air cooled toambient temperature. The seed did not exhibit substantial oxidation suchthat no spurious grain nucleation occurred in the starter cavity 12.

A nickel single crystal seed alloy comprising 27 weight a Mo and balanceNi was superheated to 2700 degrees F. and poured into mold 10 heated to2825 degrees F. in a vacuum casting furnace. The seed 20 comprised Ni-25weight % Mo and was precipitation hardened prior to placement in themold by heat treatment at 1400 degrees F. for 6 hours and air cooled toambient temperature. The seed did not exhibit substantial oxidation suchthat no spurious grain nucleation occurred in the starter cavity 12.

A nickel single crystal seed alloy comprising 27 weight % Mo and balanceNi was superheated to 2700 degrees F. and poured into mold 10 heated to2825 degrees F. in a vacuum casting furnace. The seed 20 comprised Ni-27weight % Mo and was precipitation hardened prior to placement in themold by heat treatment at 1400 degrees F. for 6 hours and air cooled toambient temperature. The seed did not exhibit substantial oxidation suchthat no spurious grain nucleation occurred in the starter cavity 12.

Although the invention has been described in detail above with respectto certain embodiments, those skilled in the art will appreciate thatmodifications, changes and the like can be made therein withoutdeparting from the spirit and scope of the invention as set forth in theappended claims.

1. (canceled)
 2. A seed for casting a single crystal article, consistingessentially of, in weight %, greater than 15.0% to about 40.0% Mo, up to0.1% C and/or B and balance essentially Ni and having hardeningprecipitates of Mo and Ni in the seed.
 3. The seed of claim 2 consistingessentially of, in weight %, greater than 15% to about 30% Mo, up toabout 0.05% C, and balance essentially Ni.
 4. A single crystal castingmold, comprising a grain starter cavity connected to an article moldcavity by a connector passage and a seed as set forth in claim 1 in saidstarter cavity wherein the seed includes the hardening precipitatestherein.
 5. A single crystal casting mold, comprising a grain startercavity connected to an article mold cavity by a connector passage and aseed in said starter cavity, said seed consisting essentially of, inweight %, greater than 15% to about 40.0% Mo, up to 0.1% C and balanceselected from the group consisting of Ni and Co wherein the seed hashardening precipitates of Mo and Ni therein.
 6. The mold of claim 5wherein said seed consists essentially of, in weight %, greater than 15%to about 30% Mo, up to about 0.05% C, and balance essentially Ni.
 7. Amethod of casting a single crystal article, comprising disposing a seedas set forth in claim 1 having the hardening precipitates therein in agrain starter cavity communicated to an article mold cavity, introducingmolten superalloy in the mold, and epitaxially nucleating and growing asingle crystal grain in said molten superalloy.
 8. The method of claim 7wherein the seed is heat treated to form the hardening precipitatestherein before being disposed in the cavity.